1. Field of the Invention
The present invention relates to ladder type filters, and more particularly, to a ladder type filter using a piezoelectric thin-film resonator.
2. Description of the Related Art
There has been an increasing demand for compact and lightweight resonators and filters using such resonators due to rapid spreading of wireless equipment such as cellular phones. In the past, dielectric filters and surface acoustic wave (SAW) filters were used. Recently, there has been a considerable activity in the research and development of a piezoelectric thin-film resonator and a filter using such a resonator.
The piezoelectric thin-film resonators may be categorized into an FBAR (Film Bulk Acoustic Resonator) type and an SMR (Solidly Mounted Resonator) type. The FBAR has a primary structure composed of an upper electrode, a piezoelectric film and a lower electrode, and a hollow space provided below the lower electrode and located within an overlapping region (resonance portion) in which the upper and lower electrodes overlap with each other across the piezoelectric film. The hollow space may be defined between the lower electrode and a silicon substrate by wet-etching a sacrificed layer on a main surface of the silicon substrate. The hollow space may also be formed by wet- or dry-etching the substrate from the backside thereof. The SMR employs an acoustic reflection film instead of the hollow space, in which a first film having a relatively high acoustic impedance and a second film having a relative low acoustic impedance are alternately laminated with a film thickness of λ/4 where λ is the wavelength of an acoustic wave of the resonator.
The upper and lower electrodes may be made of aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir) or the like. The piezoelectric thin film may be made of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconium titanate (PZT), lead titanate (PbTiO3) or the like. The substrate may be made of glass other than silicon.
The operation principles of the piezoelectric thin-film resonators will now be described. The following description is exemplarily directed to the FBAR. A voltage of a high frequency is applied between the upper electrode and the lower electrode. An acoustic wave resulting from the inverse piezoelectric effect is excited within the piezoelectric film in the resonance portion. A distortion of the piezoelectric film caused by the acoustic wave is converted into an electric signal developing between the upper electrode and the lower electrode due to the piezoelectric effect. The acoustic wave is reflected by an interface between the air and the upper electrode and another interface between the air and the lower electrode. Thus, the vertical vibration having major displacement is caused in the thickness direction of the piezoelectric film. The resonant phenomenon of the vertical vibration is utilized to form a resonator having a desired frequency response and a filter using such a resonator.
The resonance phenomenon is specifically caused at a frequency at which the total thickness H of the lower electrode, the piezoelectric film and the upper electrode (including a film added to the upper electrode) is equal to an integer multiple (n times) of half the wavelength λ (λ/2) of the acoustic wave excited. That is, the resonance phenomenon is caused at the frequency at which H=nλ/2. Assuming that V denotes the propagation velocity of the acoustic wave that depends on the material used to form the piezoelectric film, the resonance frequency F is equal to nV/(2H). This shows that the resonance frequency F can be controlled by the total thickness H of the film laminate.
A ladder filter is a typical example of the piezoelectric thin-film resonator. The ladder filter is composed of series resonators arranged between an input terminal and an output terminal and parallel resonators, and functions as a bandpass filter. FIG. 1A shows an equivalent circuit of a series resonator S, and FIG. 1B shows an equivalent circuit of a parallel resonator P. FIG. 1C shows transmission characteristics of the series resonator S and the parallel resonator P. The series resonator S has a maximum value at a resonance frequency frs and a minimum value at an anti-resonance frequency fas. The parallel resonator P has a minimum value at a resonance frequency frp and a maximum value at an anti-resonance frequency fap. FIG. 2A shows an equivalent circuit of a ladder filter composed of a single stage formed by one series resonator S and one parallel resonator P. FIG. 2B shows a bandpass characteristic of the ladder filter shown in FIG. 2A. The bandpass filter can be configured by setting the resonance frequency frs of the series resonator S and the anti-resonance frequency fap of the parallel resonator P approximately equal to each other. The anti-resonance frequency fas of the series resonator S corresponds to the frequency of an attenuation pole on the high-frequency side of the pass band, and the resonance frequency frp of the parallel resonator P corresponds to the frequency of an attenuation pole on the low-frequency side.
In order to obtain a desired pass band of the ladder filter, it is required that the resonance frequencies of the series resonator S and the parallel resonator P are designed to have a slight difference (normally, a few %). In a case where the series resonator S and the parallel resonator P are formed on an identical substrate, a frequency adjustment step (Δf adjustment step) is needed to make the series resonator S and the parallel resonator P. The center frequency of the pass band can be adjusted by adjusting the resonance frequencies of both the series resonator S and the parallel resonator P (f0 adjustment step).
Japanese Patent Application Publication No. 2005-286945 discloses a technique in which a first adjustment film and a second adjustment film are stacked on an upper electrode as a frequency adjustment film, and the first adjustment films on the series resonator S and the parallel resonator P are made different from each other in thickness. The Δf adjustment step is implemented by setting the first adjustment films on the series resonator S and the parallel resonator P different from each other in thickness, and the f0 frequency adjustment is implemented by adjusting the thickness of the second adjustment film of each of the series resonator S and the parallel resonator P. Thus, the Δf adjustment step and the f0 adjustment step can be performed separately.
Japanese Patent Application Publication No. 2006-128993 discloses that an edge of a piezoelectric film is located further in than an edge of a region in which the upper and lower electrodes overlap with each other. With this structure, it is possible to prevent the acoustic wave from leaking outwards from the resonance portion. This is called lateral leakage of acoustic wave. The structure may be formed by wet etching the piezoelectric film.
As has been described, the technique disclosed in Japanese Patent Application Publication No. 2005-286945 employs the Δf adjustment step of causing the first adjustment films of the series resonator and the parallel resonator to have different thickness values. For example, the first adjustment film of the series resonator is etched and thinned. However, it is very difficult to reliably stop etching when the first adjustment film has been etched halfway. It is thus very difficult to reliably set the difference between the resonance frequency of the series resonator and that of the parallel resonator.